Lightning strikes that hit equipment and storage or process vessels containing flammable materials can cause devastating incidents at refineries, bulk plants, processing sites and other facilities. In recent years, several incidents have occurred where lightning has struck facilities storing or handling flammable substances, which resulted in explosions and fires. Substantial monetary loss due to damage to the facility and loss of product and significant environmental damage may occur as a result of the effects of a lightning strike.
Floating roof tanks are widely used to store volatile petroleum-based liquids and limit the quantity of product evaporative emissions that may escape to the environment. Such tanks may be configured either as internal floating-roof tanks or as external floating-roof tanks. In each configuration, the floating-roof is designed to remain in contact with the product liquid surface and cover almost the entire surface of the product. A small annular area between the outermost rim of the floating roof and the inside surface of the tank shell is covered by a seal attached to the rim of the floating roof. There are many types of seals available for the annular space and are selected based on the owner's preference, the type of product, and emissions reduction requirements. In many cases today, tanks have two seals, one of which is used to reduce the emissions from the tank to very low levels.
Seals for floating roofs within storage tanks can assume a variety of different configurations. One such arrangement is shown in U.S. Pat. No. 4,308,968. That arrangement includes two different seals, the first being a primary seal and the second being a backup or secondary seal. This sealing arrangement utilizes vapor barriers in combination with flexible metal plates and wiper blades. The vapor barriers, which are common in many floating roof seals, comprise one or more layers of fabric which are generally impermeable to vapors from the liquid product stored in the tank.
One type of floating roof seal which has been found to be quite effective is the shoe seal. Shoe seals employ a shoe in the form of a series of joined-together plates which are disposed against the inner wall of the tank and which are supported by the outer rim of the floating roof. A vapor barrier extending between the outer rim and the shoe provides an effective barrier to vapors from the liquid product in the tank, inasmuch as the lower portion of the shoe extends into the product.
Examples of shoe seals are provided by U.S. Pat. No. 2,981,438; U.S. Pat. No. 3,167,206; and U.S. Pat. No. 4,130,217. In U.S. Pat. No. 2,981,438, the sealing mechanism is provided with a combination weatherhood and wax-trough. The shoe is forced against the inner tank wall by spring-loaded pistons mounted within the outer rim of the floating roof. In U.S. Pat. No. 3,167,206, the shoe is suspended from the outer rim of the floating roof by a pivoting hanger structure designed to force the shoe against the inner tank wall. In U.S. Pat. No. 4,130,217, various different members including springs are employed to maintain the shoe against the inner tank wall.
Sometimes, these sealing systems have, as an option, included a shunt located above the seals to provide an electrical path for static or lightning induced electricity from the floating roof to the tank wall so that any arcing resulting from the flow of electricity occurs near the shunt and away from the potentially ignitable vapors stored below the seals. Such a system is disclosed in U.S. Pat. Nos. 5,529,200 and 5,667,091, which are incorporated by reference herein. Another system that is similarly configured is disclosed in U.S. Pat. No. 4,371,090.
The following are excerpts from the current Chapter 6 of NFPA 780-2000, Standard for the Installation of Lightning Protection Systems, on floating roof tanks. These excerpts demonstrate the need for the improved method and apparatus of the present invention.
“6.4.1.2 Floating Roof Tanks                (a) General. Fires have occurred when lightning has struck the rims of open-top floating roof tanks where the roofs were quite high and the contents volatile. Similar above-the-seal fires have occurred when direct lightning strokes to the rims of floating roof tanks have ignited flammable vapors within the open shells. These have occurred where roofs Were low. The resulting seal fires have been at small leakage points in the seal. An effective defense against ignition by a direct stroke is a tight seal.        Fires have also occurred in the seal space of open-top floating roof tanks as a result of discharges caused by lightning. These have occurred most frequently in tanks having floating roofs and seals with vapor spaces below the flexible membranes. Similar vapor spaces will be formed where tanks are fitted with secondary seals in compliance with environmental regulations. Ignition can be from a direct stroke or from the sudden discharge of an induced (bound) charge on the floating roof, released when the charge on a cloud discharges to ground or to another cloud.        (b) Protection. Where floating roofs utilize hangers located within a vapor space, the roof shall be electrically bonded to the shoes of the seal through the most direct electrical path at intervals not greater than 10 ft (3 m) on the circumference of the tank. These shunts shall consist of flexible Type 302, 28-gauge [ 1/64 in.×2 in. (0.4 mm×51 mm)] wide stainless steel straps or the equivalent in current-carrying capacity and corrosion resistance. The metallic shoe shall be maintained in contact with the shell and without openings (such as corrosion holes) through the shoe. Tanks without a vapor space at the seal shall not require shunts at the seal. Where metallic weather shields cover the seal, they shall maintain contact with the shell.        Where a floating roof is equipped with both primary and secondary seals, the space between the two seals could contain a vapor-air mixture within the flammable range; therefore, if the design of such a seal system incorporates electrically conductive materials and a spark gap exists within that space or could be created by roof movement, shunts shall be installed so that they directly contact the tank shell above the secondary seal. The shunts shall be spaced at intervals not greater than 10 ft (3 m) and shall be constructed so that metallic contact is maintained between the floating roof and the tank shell in all operational positions of the floating roof.”        
It has been found that the shunts and shoes used in present installations do not adequately protect the floating roof storage tanks from the effects of a lightning strike. It was assumed that they had sufficient contact to the outer tank wall for conducting such a discharge; however, it has been found in some cases that the measured resistance between the shoe and outer tank wall was in the order of millions of ohms. This high resistance connection point would cause an ignition source or arcing that could ignite the flammable vapors in the primary seal, or at the secondary seal along the rim of the floating roof. American Petroleum Institute's collection of 44 years of member company data reveals 65 large aboveground storage tank fires caused by lightning, 61% of the cases evaluated (API Publication 2021A, Interim Study-Prevention and Suppression of Fires in Large Aboveground Atmospheric Storage Tanks, July 1998).
NFPA 77-2000 (Recommended Practice on Static Electricity) states that bonding of equipment or parts with less than one megohm in resistance should be sufficient to dissipate the small charges that would occur from static sources. Conductive bonding for static discharge is typically less than 10 ohms resistance. However, a lightning stroke discharge current is orders of magnitude larger that a static electricity discharge and requires a much lower bonding resistance. API RP 2003–1998 (Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents) suggests that for lightning, the bonding resistance needs to be significantly lower, no more than a few ohms. NFPA 780-2000 (Standard for the Installation of Lightning Protection Systems) requires bonding conductors be sized to be a minimum of 26,240 circular-mil cross-section copper—much less than one ohm resistance for a typically short bonding distance.
Among other factors, the present invention is based on our discovery that removing the shunts above the seals would prevent the arcing from occurring and thus prevent a rim fire. Additionally, by adding a corrosion resistant bonding strap and welding or bolting this strap to the bottom of the shoe assembly below the product level and to the lower portion of the floating roof pontoon assembly also below the product level would provide the most direct electrical path to earth for the lightning stroke current to flow and would be in an environment that is the most oxygen deficient. The use of the submerged bonding strap or shunt of the present invention would eliminate the currently used “above the seal” bonding strap and protect the tank and its combustible contents. This type of submerged bond would have less than one ohm resistance, and if any arcing occurred, it would be in a total liquid phase with no oxygen to support combustion. Furthermore, the lightning secondary effects (e.g., the induced “bound” charge as described in NFPA 780-2000) would be minimized as an added benefit.